Utilizing Mosquito Surveillance to Prevent West Nile Virus Infection--Indiana, 2012

BACKGROUND: West Nile Virus (WNV), a flavivirus, can cause encephalitis or meningoencephalitis.  From May-October 2012, the Indiana State Department of Health (ISDH) conducted mosquito surveillance to determine the emergence and extent of viral activity statewide.  Illness associated with WNV is a nationally notifiable condition that is investigated in Indiana.

METHODS: ISDH utilized entomologists, student interns, and local health department staff to collect mosquitoes from every county throughout Indiana. Female mosquitoes were collected using gravid traps placed in environmentally appropriate locations. Specimens were identified to species and grouped into pools of up to 100 mosquitoes.  Each pool was tested for WNV RNA by real-time reverse transcription polymerase chain reaction (rRT-PCR). From May –October, 137,393 mosquitoes were collected, divided into 1941 pools and tested for WNV.    Each of the state’s 92 counties was sampled 1-5 times during the season.  Positive laboratory reports indicating human infection with WNV were investigated.  All persons meeting the National Notifiable Diseases Surveillance System (NNDSS) case definition for either probable or confirmed arboviral disease (including neuroinvasive and non-neuroinvasive) were counted as cases.  The minimum infection rate (MIR) of mosquito pools was calculated and compared to the emergence of human infection.

RESULTS: Of the 1941 mosquito pools tested, WNV was confirmed in 577 (29.7%) pools. Of the 92 counties sampled, 91 (98.9%) had positive rRT-PCR results at least once during the season.  Illness investigations identified 77 human cases of WNV. Comparison of mosquito MIR with onset dates for cases revealed a rapid and steep increase in positive mosquito populations preceded an outbreak of disease.  As the MIR in mosquito populations decreased there was a corresponding decrease in the number of newly identified cases of WNV.

CONCLUSIONS: Utilizing mosquito surveillance is an effective tool for public health.  Rapid testing of specimens provides quantifiable evidence of viral activity in communities prior to the onset of illness.  This early warning allows communities to take action to prevent infection.  Health departments can use environmental information to craft public advisory messages and conduct targeted vector control activities.  Communications to residents with scientific evidence of risk can urge citizens to take precautions to prevent exposure.  Targeted vector control based on evidence of viral activity can assist communities with resource management while taking action to control infectious disease vectors.